The simple form of an adjective (or adverb): the form that is not comparative or superlative. For example, in the following sequence the first form is the absolute form: great - greater - greatest.
A very simple definition of adjectives that has sometimes been used is that they are ‘descriptive’ words. But this isn’t really very helpful. Lots of word classes can be ‘descriptive’: a noun like funeral is fairly descriptive, as is the verbleap. We might also say that the adverb quickly describes the verb ran in a sentence like He quickly ran.